The recent progress of informatization, information-communication, and multimedia technologies has created strong demands for high-density and high-capacity optical information recording media. The upper limit of the recording density of an optical information recording medium is essentially determined by a spot size of an optical beam for recording and reproducing information. An optical spot size is substantially represented as λ/NA where λ indicates the wavelength of a light source and NA indicates the numerical aperture of an object lens for forming an optical spot. As the optical spot size decreases, the recording density increases.
However, it is considered that the wavelength λ of the light source cannot be shortened beyond the wavelength of ultraviolet light due to the adsorption by an optical element and the limitation of sensitivity characteristics of a detector, and the NA is also limited by the maximum allowable tilt of the medium. On this account, there is a limit to the increase of the recording density by reducing the optical spot size.
To overcome this limit, super-resolution medium technology with which an effective optical spot size is reduced through the use of optical characteristics of a recording medium has been proposed. According to such super-resolution medium technology, an effect of masking a recording mark is produced by utilizing the changes in a temperature distribution and transmittance of a recording medium due to an optical spot thereon, so that an effective spot contributing to recording and reproduction is reduced and thus the recording/reproduction density is increased.
FIG. 21 schematically illustrates the above-mentioned medium super-resolution effect. An optical spot 111 scans a super-resolution medium relatively in the direction of an arrow 113, thus recording/reproduction being carried out. In a normal reproduction, all of the recording marks 112 within the optical spot 111 contribute to produce a reproduction signal. Meanwhile, in the case of the super-resolution medium, the optical spot 111 is masked with the exception of a central area 111a where the light is intense, and only a recording mark 112a within the central area 111a is read. This produces an effect as if an effective optical spot size contributing the reproduction is reduced. On the contrary to the example of FIG. 21, there is an alternative arrangement such that the central area 111a is masked so that a recording mark 112 in a peripheral area 111b within the optical spot 111 is read.
Conventional examples of such super-resolution medium technology are:                (1) Super-resolution reading technology using a mask with organic dye;        (2) Super-resolution technology using a photochromic mask layer; and        (3) Super-resolution technology using an inorganic oxide layer.        
Adopting an organic material as a mask layer, a medium in accordance with the methods (1) and (2) using organic dye and photochromic tend to be degraded by heat and can be read only about not more than 10,000 times, so as not to have sufficient reliability for information reproduction and thus have not been in practical use. Furthermore, due to the degradation by heat, these methods (1) and (2) cannot be adopted to produce rewritable disks.
In the meantime, regarding the super-resolution technology (3) adopting an inorganic oxide layer, Non-Patent Document 1 (Japan Journal of Applied Physics 38; (1999); p. 1656) teaches that a disk with an inorganic oxide super-resolution film can be read for not less than a million times, and a phase-change medium adopting this inorganic oxide super-resolution film is rewritable. This is because, since the super-resolution film is made of an inorganic material, the film has a heat resistance better than that of organic materials such as a mask using organic dye and a photochromic layer. For this reason, the inorganic oxide super-resolution film in accordance with the technology (3) has been prospective as a super-resolution material utilized for both read-only disks and rewritable disks.
Meanwhile, Patent Document 1 (Japanese Laid-Open Patent Application No. 2001-84643; published on Mar. 30, 2001) discloses an optical information recording medium in which a film such as a Co—Si—Na—Ca—O film and a Co3O4 film is adopted as the above-mentioned inorganic oxide super-resolution film, and the reflectance of a film stack increases as incoming light intensifies. This arrangement is contrived to tackle the following problem: If the reflectance is lowered due to the change of the complex refractive index of the inorganic oxide super-resolution film, an effective reproduction spot is broadened so that a reproduction signal amplitude characteristic enough to improve the recording density cannot be obtained.
According to Patent Document 1, moreover, the inorganic oxide super-resolution film (hereinafter, inorganic super-resolution film) has such a characteristic that the complex refractive index changes with the application of a laser beam exceeding a predetermined threshold. When this inorganic super-resolution film is adopted to an optical disk, the optical disk has a multi-layered structure, the inorganic super-resolution film being one of the multi-layers. On the occasion of playing the optical disk, the complex refractive index of the organic super-resolution film changes at a central part of the optical spot where a temperature is high, the reflectance in a complex refractive index changing area changes due to an optical multiple interference in the film stack, and as a result a signal corresponding to a part of the optical spot is enhanced and read, so that an effective spot size contributing to reproduction is reduced.
Incidentally, functions of such a film stack adopting the inorganic super-resolution film are effectively improved by increasing the range of the reflectance change of the film stack. To increase the range of the reflectance change of the film stack, it is effective to fully exploiting the optical multiple interference of the film stack.
However, according to Patent Document 1, for instance, the inorganic super-resolution film used in the patent document 1 is 50 nm thick and has a complex refractive index n−ki (i is an imaginary number) where a refractive index n before the change is 2.48 and an extinction coefficient k is 0.48, and when the incoming light intensifies, the refractive index and the extinction coefficient are changed to n=2.41 and k=0.57, respectively. When the extinction coefficient k is such a large value, it is impossible to effectively increase the range of the reflectance change in the film stack.
That is to say, when the extinction coefficient k is such a large value, light is absorbed in the process of passing through the inorganic super-resolution film, so that the inorganic super-resolution film is practically seen as a semitransparent film. Such a semitransparent inorganic super-resolution film absorbs light in the course of repeating the optical multiple interference, and hence one cannot fully exploit the optical multiple interference.
For instance, light absorption by an inorganic super-resolution film which is 50 nm thick and has an extinction coefficient k of 0.48 is examined as below. If the multiple interference is ignored in order to examine the absorption in a simple manner, the intensity of light passing through the film is represented by the following equation.I=I0×exp(−αx)where I0 is the intensity of incoming light, I is the intensity of the passing light, x is a film thickness, and α is an absorption coefficient, andα=4πk/λwhere λ is the wavelength of the incoming light.
According to this equation, as the thickness x increases, or as the extinction coefficient k increases, the intensity I of the passing light exponentially decreases.
Since the light source wavelength λ is 660 nm in this example, the transmittance (=the intensity of the passing light/the intensity of the incoming light) is 63% according to the equation above. However, in accordance with the recent demands for high-density optical information recording media, the light source wavelength has been shortened, and in this connection optical information recording media utilizing blue light with 400 nm wavelength have been in practical use. When the light source wavelength is 400 nm in the example above, the transmittance is reduced to 47%.
Note that, in the present equation the multiple interference is ignored for describing the absorption in a simple manner. When the multiple interference is taken into consideration, the light beams attenuate each other so that the transmittance is further lowered.
Thus, according to the present example, in a recent optical system with 400 nm wavelength, an amount of light is reduced to be not more than half as much as an amount of the incoming light, after only passing through the inorganic super-resolution film. For this reason, it is impossible to fully exploit the multiple interference, and this should be problematic in terms of the efficiency of the use of light.
The range of the reflectance change of the film stack can be increased by increasing the thickness of the inorganic super-resolution film so as to enhance the effects of the change of the refractive index. However, when the inorganic super-resolution film is semitransparent, light cannot pass through a thickened inorganic super-resolution film, and hence it is impractical to increase the thickness of the inorganic super-resolution film.
For instance, according to the equation above, when the thickness is doubled to 100 nm, the transmittance is significantly reduced from 63% to 40% with the light source wavelength of 660 nm, and from 47% to 22% with the light source wavelength of 400 nm. The reduction is particularly significant in short wavelengths, and this hampers the use of the multiple interference and efficient use of light.
In particular, since the efficiency of the use of light in the film stack is reduced when the inorganic super-resolution film is semitransparent, it is not possible to adopt a multi-layered recording section structure in which recording layers for recording information and recording surfaces recording information with irregularities thereon are deposited. Note that, hereinafter, a film stack (thin-film section) including a recording layer, a recording surface, and a film stack (thin-film section) contacting the recording surface are all regarded as recording sections.
To increase the reflectance change without changing the thickness of the inorganic super-resolution film, it is necessary to increase the change of the complex refractive index. However, since the change of the complex refractive index is an inherent property of a material, this approach would make little improvements.
In the meantime, patent document 2 (Japanese Laid-Open Patent Application No. 2001-189033 (Tokukai 2001-189033; published on Jul. 10, 2001)) teaches that, to maximize the reflectance change in the reproduction wavelength, a super-resolution reproduction film is modified in such a manner as to minimize the reflectance of an optical recording medium at the initial refractive index, the super-resolution reproduction film has an extinction coefficient of 0, and an interference film stack (in which films with a high refractive index and films with a low refractive index are deposited in an alternate manner) which gives rise to multiple reflections and interference in the optical recording medium is provided between the super-resolution reproduction film and a reflective film.
However, in the optical recording medium of Document 2, the multiple reflections occur through the interference film stack, thereby involving reflection due to the difference between the refractive indices at the interface of neighboring films. This decreases the efficiency of the use of light and gives rise to signal noise and failure in a servo. For this reason, the interference film stack deteriorates the signal quality.
Furthermore, the refractive index of the film and the thickness of the film cannot be easily adjusted. Thus, providing the interference film stack increases the number of manufacturing steps of the optical recording medium and increases the manufacturing costs thereof.